1. Field of the Invention
The present invention is directed to viewing screens, sometimes referred to as projection-screens, imaging-screens, or diffusion-screens. More particularly, the present invention is directed to a viewing screen having high resolution, low cost, high transmittance, low retroreflection effects, and high ambient light rejection.
2. Background of the Invention
A brief summary of screen technologies is set forth below.
Buchner (U.S. Pat. No. 997,899 and U.S. Pat. No. 1,666,808) describes daylight projection screens based on lenticular elements along with an overall absorbing filter for enhanced daylight readability.
Shimizu (U.S. Pat. No. 1,942,841) describes a lenticular screen with an absorbing filter having clear apertures for passing the projected light.
Land (U.S. Pat. No. 2,180,113) describes a non-depolarizing diffusion screen made up of an emulsion between transparent plates, wherein the emulsion consists of a plurality of light transparent media having different indices of refraction, each several microns in diameter, forming an overall layer thickness of about 10 mils. It is suggested to keep the index difference small so that the ratio of refracted to reflected light is high. Interestingly, no details are given as to why the screen does not exhibit depolarization.
MacNeille (U.S. Pat. No. 2,362,573) describes a front projection screen having a circular polarizer with clear apertures. Projected light, after being polarized, strikes a lenticular element with a reflective coating at the back side of the screen. This projected light then passes back through the clear apertures. Ambient light is first circularly polarized, and then attains the opposite polarization sense after reflection from the back side of the lenticular element. Since the ambient light is off-axis, it will not exit through the aperture, and therefore will be absorbed.
Jelley et al. (U.S. Pat. No. 2,364,369 and U.S. Pat. No. 2,380,241) describe the combination of diffuser and circular polarizer. Both surface diffusers (FIG. 4 of the '369) and volume diffusers (FIG. 5 of the '241) are described.
Staehle et al. (U.S. Pat. No. 2,378,252) describe the use of embedding spheres into a black absorptive layer that resides on a transparent substrate. Ideally, the spheres and the substrate have similar refractive indices in order to prevent total internal reflection within a sphere.
Miller (U.S. Pat. No. 3,279,314) describes an array of flat-topped conical projections, having there imposed a reflective coating (except for the flat-top), followed by either an absorptive coating on the projections, or by filling in the regions between projections with absorptive material. Additionally, the projections can have an additional diffusing element on their tips.
Northrop (U.S. Pat. No. 3,437,405) describes fibers generally aligned, running parallel to the screen surface, and embedded within a resin, providing divergence predominately in one axis.
Meyerhofer (U.S. Pat. No. 3,909,111) describes recording a three dimensional interference pattern into a gelatin film using coherent light which has passed through a diffusing medium. The resulting structure exhibits predetermined scattering characteristics. Care has been taken to avoid specifically calling this a hologram.
Zimmerman et al. (U.S. Pat. No. 5,481,385) utilize an approach similar to Miller, however the conical projections operate on the principal of total internal reflection (TIR) by way of a low index fill containing black particles.
Petersen et al. (U.S. Pat. No. 5,609,939) describe recording a three dimensional interference pattern into a photosensitive film using coherent light that has passed through a holographic diffuser. The resulting surface structure exhibits controllable scattering characteristics and very high resolution properties.
Abileah et al. (U.S. Pat. No. 5,629,784) describe a direct view liquid crystal display, wherein films are placed on the viewer-side of the liquid crystals, either interior or exterior to the front polarizer (i.e. analyzer). The film stack comprises a refracting film having facets, and thereafter an optional diffuser. The diffuser can have a rough surface facing the viewer, or can be of the holographic type.
Larson (U.S. Pat. No. 5,751,388) describes a front projection screen (FIG. 9 and Col. 13, lines 34-52) using a polarization sensitive scattering element (PSSE) to preferentially diffuse the polarized light from a projector, while absorbing ambient light of the opposite polarization.
Clabborn (U.S. Pat. No. 6,123,877) describes the fabrication of a symmetrical diffuser followed by stretching to provide asymmetrical viewing angles.
Chou et al. (U.S. Pat. No. 6,163,402) describe the use of a volume diffuser and a linear polarizer, whereby the diffuser passes a portion of light without changing the incident polarization, and substantially depolarizes incident light that is laterally scattered, which is subsequently absorbed in the polarizer, thereby minimizing loss of resolution. The volume diffuser is constructed from particles dispersed within a binder. A laminate is proposed having AR and anti-smudge coatings in the front, the diffuser towards the rear, and either a matte surface or AR coating on the surface where projected light is incident.
Allen et al. (U.S. Pat. No. 6,239,907) describe the construction of a rear projection screen by use of a dispersive birefringent element to independently control the amount of divergence in each axis.
Harada et al. (U.S. Pat. No. 6,381,068) describes the construction of a front projection screen utilizing a reflective polarizer element in combination with a diffusing element and/or a glare suppression element.
While the application of screens for projection applications are generally well understood, some background information is necessary to understand direct-view applications, where a collimated backlight and a front screen enables a liquid crystal display to have CRT-like viewing angle performance.
Direct-View LCDs Using Collimated Light and Front Screens
Fischer (U.S. Pat. No. 3,840,695) describes a liquid crystal display that utilizes a light scattering film or foil above the analyzer (i.e. closest to the viewer) enabling wide angles when used in combination with collimated backlight (e.g. 3M louver film and a fluorescent lamp).
Bigelow (U.S. Pat. No. 4,171,874) details an arrangement similar to Fischer, except a point source of light is used.
Zimmerman et al. (U.S. Pat. No. 5,481,385) and Abileah et al (U.S. Pat. No. 5,629,784 ca. 1997), both referenced previously, describe direct-view liquid crystal displays, employing a collimated backlight and a front screen. Yamaguchi (U.S. Pat. No. 6,421,103) describes the use of a collimating plate in a direct-view system. The specification also describes the nuances related to the desired degree of collimation for a given pixel-pitch.
For very high resolution direct view applications, such as a 10.4″ diagonal LCD with XGA resolution (1024×768 color pixels), a high degree of collimation is required to avoid loss of resolution caused by the mixing of adjacent pixel information in the screen. Further, if the application requires high brightness and a degree of compactness, the concept of etendue must be carefully considered. Such was the case in U.S. Pat. No. 6,428,198. The '198 patent details a compact, high brightness system employing a point source, collection optics, fiber optics, a dimmer, homogenizer, more fiber optics, non-imaging “morphing” collimators, a turn-the-corner assembly feeding a waveguide that illuminates a liquid crystal display capped with a viewing screen. Etendue was carefully considered in the design in order to maintain high brightness in a compact assembly. U.S. Pat. No. 6,428,198 is incorporated herein by reference.
In laboratory testing of a device in accordance with the '198 patent, it has been observed that existing screen technology (e.g. black-matrix beaded-screens and engineered surface-diffusers), was not suitable for use in this application, exhibiting one or more deleterious effects, such as high absorption, high ambient light reflectance, high retroreflectance, “noisy” imagery, etc. Some of these effects are described and characterized in “Image noise in high-resolution rear-projection screens”, B. Larson, et al., Proc. SPIE Vol. 4712, p. 202-211, Cockpit Displays IX: Displays for Defense Applications; Darrel G. Hopper; Ed., August 2002.
It is with this experience that the need for an improved, manufacturable screen technology was realized.